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Thursday, 11 July 2019

New Climate Data Points To Mass Extermination

Carbon
dioxide emissions may trigger a reflex in the carbon cycle, with devastating
consequences, study finds.

In the brain,
when neurons fire off electrical signals to their neighbors, this happens
through an “all-or-none” response. The signal only happens once conditions in
the cell breach a certain threshold.

Now an MIT
researcher has observed a similar phenomenon in a completely different system:
Earth’s carbon cycle.

Daniel
Rothman, professor of geophysics and co-director of the Lorenz Center in MIT’s
Department of Earth, Atmospheric and Planetary Sciences, has found that when
the rate at which carbon dioxide enters the oceans pushes past a certain
threshold — whether as the result of a sudden burst or a slow, steady influx —
the Earth may respond with a runaway cascade of chemical feedbacks, leading to
extreme ocean acidification that dramatically amplifies the effects of the
original trigger.

This global
reflex causes huge changes in the amount of carbon contained in the Earth’s
oceans, and geologists can see evidence of these changes in layers of sediments
preserved over hundreds of millions of years.

Rothman
looked through these geologic records and observed that over the last 540
million years, the ocean’s store of carbon changed abruptly, then recovered,
dozens of times in a fashion similar to the abrupt nature of a neuron spike.
This “excitation” of the carbon cycle occurred most dramatically near the time
of four of the five great mass extinctions in Earth’s history.

Scientists
have attributed various triggers to these events, and they have assumed that
the changes in ocean carbon that followed were proportional to the initial
trigger — for instance, the smaller the trigger, the smaller the environmental
fallout.

But Rothman
says that’s not the case. It didn’t matter what initially caused the events;
for roughly half the disruptions in his database, once they were set in motion,
the rate at which carbon increased was essentially the same.Their characteristic rate is likely a
property of the carbon cycle itself — not the triggers, because different
triggers would operate at different rates.

What does this
all have to do with our modern-day climate? Today’s oceans are absorbing carbon
about an order of magnitude faster than the worst case in the geologic record —
the end-Permian extinction. But humans have only been pumping carbon dioxide
into the atmosphere for hundreds of years, versus the tens of thousands of
years or more that it took for volcanic eruptions or other disturbances to
trigger the great environmental disruptions of the past. Might the modern
increase of carbon be too brief to excite a major disruption?

According to
Rothman, today we are “at the precipice of excitation,” and if it occurs, the
resulting spike — as evidenced through ocean acidification, species die-offs,
and more — is likely to be similar to past global catastrophes.

“Once we’re
over the threshold, how we got there may not matter,” says Rothman, who is
publishing his results this week in the Proceedings of the National Academy of
Sciences.“Once you get over it, you’re dealing with how the Earth works, and it
goes on its own ride.”

A carbon feedback

In 2017,
Rothman made a dire prediction: By the end of this century, the planet is
likely to reach a critical threshold, based on the rapid rate at which humans
are adding carbon dioxide to the atmosphere. When we cross that threshold, we
are likely to set in motion a freight train of consequences, potentially
culminating in the Earth’s sixth mass extinction.

Rothman has
since sought to better understand this prediction, and more generally, the way
in which the carbon cycle responds once it’s pushed past a critical threshold.
In the new paper, he has developed a simple mathematical model to represent the
carbon cycle in the Earth’s upper ocean and how it might behave when this
threshold is crossed.

Scientists
know that when carbon dioxide from the atmosphere dissolves in seawater, it not
only makes the oceans more acidic, but it also decreases the concentration of
carbonate ions. When the carbonate ion concentration falls below a threshold,
shells made of calcium carbonate dissolve. Organisms that make them fare poorly
in such harsh conditions.

Shells, in
addition to protecting marine life, provide a “ballast effect,” weighing
organisms down and enabling them to sink to the ocean floor along with detrital
organic carbon, effectively removing carbon dioxide from the upper ocean. But
in a world of increasing carbon dioxide, fewer calcifying organisms should mean
less carbon dioxide is removed.

“It’s a
positive feedback,” Rothman says. “More carbon dioxide leads to more carbon
dioxide. The question from a mathematical point of view is, is such a feedback
enough to render the system unstable?”

“An inexorable rise”

Rothman
captured this positive feedback in his new model, which comprises two
differential equations that describe interactions between the various chemical
constituents in the upper ocean. He then observed how the model responded as he
pumped additional carbon dioxide into the system, at different rates and
amounts.

He found that
no matter the rate at which he added carbon dioxide to an already stable
system, the carbon cycle in the upper ocean remained stable. In response to
modest perturbations, the carbon cycle would go temporarily out of whack and
experience a brief period of mild ocean acidification, but it would always
return to its original state rather than oscillating into a new equilibrium.

When he
introduced carbon dioxide at greater rates, he found that once the levels
crossed a critical threshold, the carbon cycle reacted with a cascade of
positive feedbacks that magnified the original trigger, causing the entire
system to spike, in the form of severe ocean acidification. The system did,
eventually, return to equilibrium, after tens of thousands of years in today’s
oceans — an indication that, despite a violent reaction, the carbon cycle will
resume its steady state.

This pattern
matches the geological record, Rothman found. The characteristic rate exhibited
by half his database results from excitations above, but near, the threshold.
Environmental disruptions associated with mass extinction are outliers — they
represent excitations well beyond the threshold. At least three of those cases
may be related to sustained massive volcanism.

“When you go
past a threshold, you get a free kick from the system responding by itself,”
Rothman explains. “The system is on an inexorable rise. This is what
excitability is, and how a neuron works too.”

Although
carbon is entering the oceans today at an unprecedented rate, it is doing so
over a geologically brief time. Rothman’s model predicts that the two effects
cancel: Faster rates bring us closer to the threshold, but shorter durations
move us away. Insofar as the threshold is concerned, the modern world is in
roughly the same place it was during longer periods of massive volcanism.

In other
words, if today’s human-induced emissions cross the threshold and continue
beyond it, as Rothman predicts they soon will, the consequences may be just as
severe as what the Earth experienced during its previous mass extinctions.

“It’s
difficult to know how things will end up given what’s happening today,” Rothman
says. “But we’re probably close to a critical threshold. Any spike would reach
its maximum after about 10,000 years. Hopefully that would give us time to find
a solution.”

“We already
know that our CO2-emitting actions will have consequences for many millennia,”
says Timothy Lenton, professor of climate change and earth systems science at
the University of Exeter. “This study suggests those consequences could be much
more dramatic than previously expected. If we push the Earth system too far, then
it takes over and determines its own response — past that point there will be
little we can do about it.”

This research
was supported, in part, by NASA and the National Science Foundation.

1 comment:

These are not opinions, they are mere facts. To those that strive to put profit above human life of course it would be opinion. But as the author states, once we get above the threshold it won't matter because we'll all become extinct and all the money in the world won't be enough to solve the problem.